Key Considerations for Epoxy and
Urethane Casting
Systems
There are a number of considerations for selecting the best
material to provide a solid, bubble free, crack free, functional and
cosmetically pleasing epoxy or urethane casting. The key material properties are:
-
Mixed viscosity
-
Reactivity
-
Exotherm
-
Shrinkage
-
Thermal expansion
-
Thermal shock
-
Thermal stability
-
Thermal conductivity
The above properties are especially important when casting
electrical and electronic components using either epoxy or polyurethane
compounds.
-
Mixed Viscosity
The
lower the mixed viscosity the easier it is to process the material. Materials
that contain fillers to impart certain cured properties are higher in
viscosity than those that are unfilled. Viscosity can be decreased by the
application of heat. Reducing the viscosity by heating can be achieved by
heating the resin and hardener components separately or by heating the mixture
after the resin and hardener are mixed together in the specified ratio.
It is
preferable to heat the individual components since heating the mixture will
result in a shortened pot life. As a "rule of thumb", the pot life will be
reduced by 50% for every 100C rise in temperature when heating
the mixed material.
Some
epoxy and/or urethane casting systems include highly filled, high viscosity resins and un-filled low
viscosity hardeners which, when mixed in the recommended ratio, yield quite
acceptable mixed viscosities. The best range of mixed viscosity will depend on
the application.
The
lower the mixed viscosity the better.
-
Reactivity
The
reactivity of Epoxy and Polyurethane compounds will depend on the type of
hardener employed and the chemistry involved. All reacting (curing) materials
generate heat as a by-product of the reaction. The amount of heat generated
will depend on the chemistry involved and can reach very high temperatures.
for example; epoxy systems formulated for thin film bonding applications can
generate enough energy to self-ignite in large mass castings.
The
un-dissipated heat generated as a by-product of the curing process will intern
speed the reaction further until the material solidifies. Depending on the
chemistry involved, systems that are designed to be heat cured, will generate
negligible amounts of exotherm and require the application of external energy
(heat) to commence the reaction. These materials have very long pot lives.
As a
general rule, urethane compounds generate less heat during cure then epoxies.
Pot
life is defined as the period of time, commencing from the time the resin and
hardener are mixed together, the mixture remains pourable in its intended
application. As a rule of thumb, the faster the reaction the higher the
exotherm.
The
slower the reaction the better.
-
Exotherm
Exotherm is defined as the increase in temperature above the cure temperature
due to the energy released by the reaction. Excessive exotherm can damage
components especially in encapsulating electronic circuits. If the resin and
hardener are heated to lower the mixed viscosity, the resultant greater
reactivity will cause the exotherm (heat) to be generated in a shorter period
of time. The ultimate temperature will be much higher as there is less time
for the mix to dissipate the internal heat being generated.
High
reactivity hardeners used in large mass casting can result in "runaway
exotherm" because the heat being generated can not be dissipated at a
sufficient rates from the center of the mass. In extreme cases, the
temperature in the center of the mass can reach extremes to the point where it
actually chars or even explodes.
The
lower the exotherm the better. Use low reactivity hardeners for large mass
castings and faster reactivity hardeners for small mass casting or thin film
applications.
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-
Shrinkage
Shrinkage is the reduction in volume or linear dimensions as a result of cure.
Excessive shrinkage will result in damage to embedded components and residual
built in stresses in the casting. Built in stresses make the finished casting
prone to cracking.
As a
rule, un-filled products shrink more than those containing fillers and slow
reactivity materials will shrink less than high reactivity systems. In most
cases, the higher the filler content the lower the shrinkage and the better
the thermal conductivity of the casting.
Polyurethane compounds may shrink less and having better elongation properties
may exert less pressure on embedded components.
The
lower the shrinkage the better.
-
Thermal Expansion
Thermal
expansion is a function of the chemistry employed and the filler loading of
the system. The higher the filler loading the lower the thermal expansion. In
general, more flexible resin systems will exhibit higher thermal expansion
properties.
The
thermal expansion properties should be as close as possible to those exhibited
by the rest of the components in the casting.
-
Thermal
Shock
The
ability to withstand thermal shock is generally a function of flexibility. The
more flexible the cured system the better its ability to withstand thermal
shock. This can be a problem if the potted component is required to operate at
elevated temperatures because many flexibilized casting systems are not well
suited to high temperature operation. The most suitable casting systems are
those that possess a good combination of toughness and flexibility.
Depending on the application, the most thermal shock resistant material can be
developed through formulating techniques to yield the required combination of
shrinkage, tensile strength, elongation and thermal expansion for the part in
question.
-
Thermal Stability
Thermal
stability is determined by the ability of a given casting system to maintain a
certain set of minimum cured properties at elevated temperatures for a given
period of time. Most casting systems will experience progressive loss of
strength and overall reduction in properties as they age at elevated
temperatures. The detrimental effects of the loss of properties on the
component performance will be determined by the demands placed on the part in
service.
For
example; if a component is not subjected to any mechanical stresses in
service, an encapsulant with a lower set of minimum properties at elevated
temperature would be very satisfactory. On the other hand, if the service
conditions demand sustaining mechanical or internal stresses, the required
minimum properties to be maintained by the encapsulant will be much higher.
The
operating temperature capabilities of a given material will be dependent on
the demands placed upon the encapsulated component in service.
-
Thermal Conductivity
Unfilled Epoxy and Polyurethane systems are, by their nature, excellent
insulators which means that they are relatively poor at conducting heat.
Thermal conductivity can be greatly improved by incorporating certain types of
fillers. The higher the filler loading the better the resultant thermal
conductivity. Formulations have been developed to maximize heat transfer
through the casting system.
Highly
filled systems are also high in mixed viscosity and usually require heating
for ease of processing.
The
higher the filler content the better the thermal conductivity.
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Many of
the key properties listed above are somewhat opposing one another and
represent a set of trade-offs and compromises in developing the correct
material for a given application. It is important to consider all the service
requirements for a given component in order to find the correct combination of
cured properties to satisfy those requirements.
Disclaimer:
The above information is general in nature and
is based solely on experiences by Crosslink Technology Inc. The recommendations
provided herein may not be applicable in all situations. They are provided to
the recipient as part of our customer service and the user must determine the
relevance of the information to his/her application, considering any limitations
that may be applicable thereto. Crosslink technology Inc. does not accept any
liability for direct or consequential damages resulting from the implementations of these recommendations or the use of this information.
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